The Coanda effect as used in air flow amplifiers to achieve high amplification ratios is well known. As disclosed in U.S. Pat. No. 2,052,869, the Coanda effect involves discharging a small volume of fluid (primary fluid) under high velocity from a nozzle, the nozzle being immediately adjacent a shaped surface. The primary fluid tends to follow the shaped surface and as it does so it induces surrounding fluid (secondary fluid) to flow with it. In an air flow amplifier, a small volume of primary fluid is therefore used to move a much larger volume of secondary fluid, the amplification ratio being the total volume of primary and secondary fluid discharged from the device in relation to the volume of primary fluid supplied.
Other types of fluid or air moving devices, commonly called ejectors, are also well known. In general, such ejectors have been used to create relatively high suction and have therefore been used effectively as pumps. Characteristically, such ejectors are capable of only limited air flow amplification but that failing is not of major concern in a device intended primarily to generate high suction. Where high amplification has been required, Coanda-type amplifiers have been available and have generally been regarded as the more effective means for achieving high amplification ratios.
Unfortunately, air flow amplifiers which operate on the Coanda principle do have certain disadvantages. Since some of the kinetic energy in the primary stream must be used to turn that stream (and also a part of the secondary stream), the Coanda profile must be machined carefully for optimum performance. Also, Coanda amplifiers are particularly sensitive to back pressure at the outlet and, as this pressure is increased, it can cause a sudden detachment of the primary stream from the profile, resulting in turbulence and flow reversal in the suction inlet area. Efforts to reduce the flow reversal characteristics, so that reversal occurs only at higher exit pressures, have had the effect of substantially reducing the amplification ratios (see, for example, U.S. Pat. No. 3,801,020).
The term "high amplification ratio" is used herein to mean ratios of 10:1 or better. A well-designed and fabricated Coanda amplifier might achieve amplification ratios of 15:1, for example. By contrast, a typical ejector of the type used to create a vacuum in steam condensers would be expected to have an amplification ratio in the order of about 3:1.
Other references illustrating the state of the art are U.S. Pat. Nos. 2,713,510, 2,920,448, 3,047,208, 2,120,563, and 3,795,367.